This invention relates to digitally encoding and decoding electrical signals, e.g., to efficiently store music on magnetic tape.
In a continuously variable slope delta (CVSD) encoding system, the encoded signal represents the slopes of individual line segments which together approximate the input signal, and is generated by determining, for each segment, the difference between the input signal and a reference signal whose value is determined by the previous segments of the input signal. In the most rudimentary delta encoding system, the digital output of the encoder represents, at sequential points in time, whether the input signal is greater or less than the reference, and the reference is increased or decreased, at a constant rate, accordingly.
To improve the accuracy of the encoding and reduce the quantization noise introduced by the encoding process, a CVSD system increases and decreases the reference signal at a variable rate corresponding as closely as possible to the magnitude of the instantaneous slope of the input signal.
The Harris, Inc. HR-3210 and the Motorola, Inc. XC3417/18 modulators/demodulators have syllabic filters examining the digital output to detect continuous series of 1's or 0's, indicating a too great or too small rate of change of the reference, and modifying that rate accordingly. In De Boer et al. U.S. Pat. No. 3,490,045, the digital output is integrated to obtain a voltage representing the relative occurrence of 1's and 0's in the output and, if the resulting voltage exceeds a threshold, the rate of change of the reference is changed accordingly. Linder U.S. Pat. No. 3,109,987 and Gaunt U.S. Pat. No. 3,582,784 show encoders wherein the previous rate of change of the reference is detected and utilized to determine the future rate of change.